Provided are high intensity discharge (HID) light sources that provide high efficiency and quality illumination via tight thermal control of the light sources without adversely heating the environment surrounding the light sources. Applications for the high intensity discharge light sources provided herein include agriculture, where increased efficiency of light application provides the functional benefit of improved growth characteristics, such as higher plant yields, while avoiding high temperatures that would otherwise hinder plant growth.
High intensity discharge light sources generate a significant amount of heat and also have a spectral output that is temperature dependent. This can complicate efforts to cool light sources in applications where heat build-up around the bulb is desirably avoided. For example, if a HID light source is actively cooled, the bulb operating temperature may be reduced with an attendant spectral output change that is no longer optimal. In agricultural applications, a well-controlled spectral output is important for achieving optimal plant growth, but it is important that the temperature of the environment in which the plants grow is not too high. Heat build-up, however, associated with use of one or more HID light sources adversely impacts plant growth, particularly for plants positioned immediately below the HID light source. Furthermore, the heat generated from the bulb can cause a bulb melt down or explosion, resulting in debris falling onto a plant canopy or other flammable material below the bulb, which in turn could result in fire.
Conventional systems that attempt to address certain of the above-referenced problems in the art suffer from fundamental disadvantages, including light fixtures that simply allow heat to escape to the environment. For example, U.S. Pat. No. 7,456,556 describes an outer envelope that is a tube of quartz or glass around a lamp, wherein in the case of lamp failure or explosion the outer envelope may contain glass pieces that fall and avoid injury. That tube, however, is configured to provide heating to the environment and to not trap heat inside the envelope, and accordingly is fundamentally incompatible with light sources having an insulative sleeve that traps heat next to the bulb to maintain a relatively high operating temperature.
U.S. Pat. No. 6,247,830 describes a cylindrical transparent sleeve for agricultural light bulbs. That system, however, dissipates heat from the light bulb by pushing or pulling air through the sleeve. This directly cools the bulb, having an adverse impact on the ideal operating temperature of the bulb and an attendant decrease in spectral output.
U.S. Pat. No. 7,245,081 describes a high intensity discharge lamp having a transparent outer jacket. That outer jacket, however, is to seal the arc tube from the outside environment and, therefore, is an integral part of the bulb, such as corresponding to the bulb outer surface. There is no teaching or suggestion of insulating the resultant bulb to control the bulk bulb operating temperature, such as under cooling conditions.
U.S. Pat. No. 7,083,309 describes a lamp with an integrated outer transparent tube. However, there is no teaching or suggestion as to the tube being thermally insulative, and the bulbs described therein are fluorescent lamps.
U.S. Pat. No. 8,040,031 describes a lamp with an outer bulb. That outer bulb, however, is to protect the lamp main body against external damage, prevent scattering of flakes due to breakage of the main body of the lamp, and decrease emission of ultra-violet radiation. There is no teaching or suggestion as to use of a thermally insulative sleeve to improve bulb spectral output.
There is a need in the art for a light source that can be operated at optimal efficiency by tightly controlling the operating temperature of the light source, including at relatively high operating temperature, without risking harm to the environment below such as by an unwanted temperature increase and/or risk of fire or injury in the event of a light source failure.